MMO stands for Massive Multiplayer Online (Massive Online Multiplayer, in literal translation) and represents a online game mode, where multiple players compete and interact at the same time.
Unlike the multiplayer games (multiplayer), where the number of players is limited to a maximum of half a dozen, MMO games allow tens, hundreds or even thousands of people to interact at the same time.
MMO games arose from PC's connectivity to the internet, that is, the first games were developed for computers. However, with the evolution of consoles, MMO games are now part of new video game models, which have connectivity to the world wide web.
MMO and MMORPG
One MMORPG (Massive Multiplayer Online Role Playing Game) can be considered an MMO, however an MMO cannot always be an MMORPG.
While the MMO is based on simultaneous multiplayer games, the MMORPG, in addition to having this feature, also has the same attributes as a roleplaying (Role Playing Game).
RPG is a game format that does not have a chronological order or pre-defined actions. The player is free to "explore" the entire virtual world of the game, do alternative missions and customize his character as the game progresses.
The MMORPG is, therefore, the combination of MMO and RPG formats.
See also the meaning of MOB.